Use VSEPR theory to predict the geometric shapes of the...

Key Concepts

Multiple Bonds as a Single Domain

In VSEPR theory, multiple bonds (double or triple bonds) are treated as a single electron domain when considering the steric number and overall electron domain geometry. This simplification allows chemists to use VSEPR theory to predict molecular shapes without having to differentiate between the number of electrons in each bond type, focusing solely on the spatial arrangement of electron domains.

Bonding and Lone Pairs

In predicting molecular geometry using VSEPR theory, it is important to distinguish between bonding pairs and lone pairs of electrons. Bonding pairs are shared between atoms and form chemical bonds, while lone pairs are unshared and occupy more space due to their higher electron density. The presence of lone pairs can result in molecular geometries that deviate from the ideal electron-pair geometry, leading to shapes that are slightly distorted.

Steric Number

The steric number is the total count of electron domains, including both bonding pairs and lone pairs, surrounding the central atom. This numerical value helps in predicting the basic geometric arrangement of electron pairs around the atom, which in turn influences the molecular shape. For instance, a steric number of four generally leads to a tetrahedral electron domain geometry.

Electron Domains

Electron domains refer to regions around a central atom where electrons are most likely to be found. These regions include bonding pairs (from single, double, or triple bonds, which each count as one domain) and lone pairs of electrons. In the context of VSEPR theory, identifying the number of electron domains is essential for determining the molecule’s electron-pair geometry.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules based on the repulsions between electron pairs in the valence shell of the central atom. The theory assumes that electron pairs, whether bonding or nonbonding, will arrange themselves as far apart as possible to minimize repulsion, thereby determining the overall three?dimensional shape of a molecule.

Transcript

00:01 We will be using the vsepr theory to predict the geometric shapes, and i have written a simplified version of this here.

00:08 And so when i say one is a lone pair, this means that it is included in the total of the three electron groups.

00:15 Another example of this is if there were two lone pairs and included in the four electron groups, and so therefore it would be a bent geometric shape.

00:25 And so let's go ahead and look at our first example and work through these.

00:29 The first thing that we're going to do is calculate the total number of valence electrons.

00:34 So with two nitrogen atoms, we know that nitrogen has five valence electrons, and we know there is two nitrogen atoms, and so this gives us a total of 10 valence electrons.

00:45 And so using this, we will know that there are 10 valence electrons total.

00:52 So we can assume that each nitrogen atoms will have five pairs.

00:58 And so one, two, three, four, five.

01:06 And so based on this, we are going to look at this nitrogen atom.

01:11 You can look at this one too.

01:12 It will be exactly the same.

01:14 And you can see that there is an electron group here.

01:17 And there is an electron group here.

01:19 You have to count for the entirety of the electrons.

01:24 It doesn't matter if there is, you know, six or nine or so forth.

01:30 It's the entire group.

01:31 And so this one will be linear because there are two electron groups surrounding this nitrogen atom.

01:41 And so now let's go ahead and move on to our next one, which is hcn.

01:46 And let's first calculate the number of valence electrons.

01:49 So with hydrogen, there's one.

01:51 With carbon, there's four.

01:53 And then with nitrogen, there's five, which gives us a total of 10.

01:56 And so we know hydrogen only has one valence electron, and so there will only be one bond here.

02:05 And then, of course, there will be three bonds here.

02:08 And this gives us a total of 10 valence electrons.

02:12 And so we are going to look at our central atom here, which is the carbon.

02:17 And what we are going to do is see how many electron groups are surrounding this.

02:21 And so just like before, there are only two groups of electrons surrounding this.

02:27 This carbon, and so therefore, again, it is linear.

02:32 Okay, so now for our next one, we have nh4, and this one does have a plus one charge, so that's something to keep in mind when we're calculating the total of valence electrons.

02:40 And so here for the valence electrons, we have four hydrogens, each with a valence electrons of one, and then we have one nitrogen, and nitrogen has a valence electron of five.

02:53 We can see this from the periodic table, and so this gives us our total of nine.

02:58 But because we have this plus one charge up here, we have to subtract.

03:06 And so we have to subtract one because we have one less electron because of this.

03:11 And so now this brings down our total to eight, which means we will have four pairs of electrons surrounding this nitrogen atom...

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